Programming Covalent Organic Frameworks for Photocatalysis: Investigation of Chemical and Structural Variations

doi:10.1016/j.matt.2019.10.026

Matter

Volume 2, Issue 2, 5 February 2020, Pages 416-427

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Highlights

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The modularity of COFs allows accelerated photocatalyst design and discovery
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Topology is crucial for a longer charge-carrier lifetime of COFs
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Electronic properties of the paired struts affect the COFs' optical properties
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Improving the materials' crystallinity allows for extended photoadsorption

Progress and Potential

Harnessing the energy of light to trigger chemical reactions has attracted considerable interest. Development of efficient photosensitizers through direct molecule engineering is often synthetically challenging. Advancements in reticular chemistry have enhanced our ability to design and develop materials tailor-made for use in a wide variety of applications. Here, we demonstrate how the two-dimensional covalent organic frameworks (COFs) display the appropriate combination of properties to serve as a scaffold for photocatalyst design. We conducted a series of comparative studies detailing the design principles for the achievement of highly efficient photocatalysts. These descriptors allow us to establish the first relationships between the composition, crystallinity, lattice symmetry, and photoreactivity for this class of materials. We anticipate that our work will guide future efforts to make COFs suitable for practical photocatalytic transformations and related applications.

Summary

There has been a surge of interest in light-driven chemical transformations using organic photoredox catalysts, whereby a suitable energy alignment and efficient exciton migration are crucial for achieving high efficiency. We show here that covalent organic frameworks (COFs) offer the flexibility required to be ideal platforms for photocatalyst design, lending themselves to fine control over photoreactivity. To understand the factors that dictate photoreactivity, we explore a comparative study of a series of porous materials built with amine bonds with varied composition, crystallinity, and topology, in the efficiency of photogenerated reactive oxygen species. Combined spectroscopies, density functional theory calculations, and catalytic evaluations revealed an essential interplay among these parameters and the photoreactivity of the resulting materials. Beyond basic considerations, such as spectral absorption and crystallinity, the material's lattice symmetry has a significant impact on photoreactivity. We anticipate that the structure-performance relationships developed will provide a robust rule of thumb for designing COF-based photocatalysts.

Graphical Abstract

Material Advancement Progression

MAP3: Understanding

Keywords

covalent organic frameworks

porous organic photosensitizers

photocatalysis

photoredox catalysts

reactive oxygen species

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